There have conventionally been considered counterplans for improvement of an occupant protecting ability at the time of a side surface impact (hereinafter, "side impact") of a vehicle. In recent years, the building-in of a side-impact air bag device at a side portion of a front seat of a vehicle has been promoted. A structure of this type of front seat having a side-impact air bag device built-in is disclosed in Japanese Patent Application Laid-Open (JP-A) No. 6-64491. The structure disclosed therein will be described hereinafter, focusing on the relation between the seat outer cover and the air bag device.
FIG. 8 shows a side view of a front seat 150 having an air bag device built-in and disclosed in the above prior art. As shown in this figure, a side-impact air bag device 154 is built-in in a side portion, toward a door, of a seat back 152 of the front seat 150. The side-impact air bag device 154 is structured to basically include a sensor 157 disposed below a seat cushion 156 and detecting a state of a large load being applied to a side door (not shown), a substantially cylindrical inflator 158 connected to the sensor 157 and ejecting gas by the sensor 157 detecting a state in which a large load is applied, and a bag 160 which inflates due to gas ejected from the inflator 158. Further, as shown in FIG. 9, the inflator 158 and the bag 160 are accommodated in an internal space formed by an air bag case 164 and a lid 166 which are fixed to a side portion, toward the door, of a seat back frame 162.
Further, in the structure disclosed in the above prior art, an outer cover 168 of the seat back 152 is formed by stitching a plurality of divided portions so as to allow an opening movement of the lid 166 at the time the bag inflates. Namely, the outer cover 168 of the seat back 152 is divided into a front main outer cover 170 which covers a front surface of a central portion of the seat back 152, two front side outer covers 172, 174 which cover respective front sides of side portions of the seat back 152, a seat back side portion outer cover 176 which covers the lid 166 of the air bag device 154, and a rear outer cover 178 which covers a back side of the seat back 152.
In order to allow opening of the seat back side portion outer cover 176 located at the side of the lid 166, a front end portion 176A of the seat back side portion outer cover 176 and a rear end portion 174A of the front side outer cover 174 are sewn together by a sewing thread 180. The sewing strength at the above sewn-together portion is set to be low by sewing by single stitching as shown in FIG. 9. Accordingly, this sewn-together portion is used as a rupture portion. In contrast, a rear end portion 176B of the seat back side portion outer cover 176 and a front end portion 178A of the rear outer cover 178 are each formed into a loop and are stitched by using sewing threads 182, 184, and further, the loop-shaped rear end portion 176B and front end portion 178A are sewn together by a sewing thread 186. The sewing strength at this sewn-together portion is set to be high by sewing by double stitching as shown in FIG. 9. Accordingly, this sewn-together portion does not rupture, and serves as a hinge used for the opening movement of the seat back side portion outer cover 176.
Accordingly, when the bag 160 unfolds due to gas ejected from the inflator 158, the lid 166 opens toward an outer side of the seat (i.e., in the direction indicated by arrow W in FIG. 9) around a hinge portion 166A formed at the rear end of the lid 166. Due to the opening force of the lid 166, the portion where the front end portion 176A of the seat back side portion outer cover 176 and the rear end portion 174A of the front side outer cover 174 are sewn together is broken.
However, in this seat structure having a side air bag device, an insert wire 192 is embedded in a seat back pad 190 in the vicinity of an occupant supporting surface (top plate portion) 190A. A sewn-together portion of the front main outer cover 170 and the front side outer cover 172 is engaged with the insert wire 192 via a hanging member 188 for bringing forth the designed seat configuration. Accordingly, at an initial stage of unfolding of the bag 160, a pull-out load acts on the insert wire 192 due to reactive force of the seat outer cover 168 caused by the unfolding force of the bag 160. When the pull-out load increases and the insert wire 192 is thereby pulled out from the seat back pad 190, the outer cover 168 extends greatly outward in the transverse direction of the seat and toward the front of the vehicle as indicated by the two-dot chain line in FIG. 9. For this reason, the sewn-together portion formed by the sewing thread 180, which was originally intended to be broken, is difficult to break. Accordingly, the unfolding speed of the bag 160 may decrease or the unfolded form of the bag 160 may be unstable.
As art related to the present invention, Japanese Patent Application Laid-Open (JP-A) No. 9-76868 (laid-open on Mar. 25, 1997) discloses an end of a front-side seat outer cover of a sewn-together portion to be broken and a seat-side fixing member being connected together by a strong cloth.